A number of methods are known from the patent literature for improving the toughness properties of polyacetals, by incorporating crosslinked or uncrosslinked elastomers, in some cases also grafted elastomers. The following may be mentioned as examples: POM modified with polyurethanes (German Patent No. 1,193,240), POM modified with a graft copolymer prepared on an acrylic ester/butadiene basis (German Patent No. 1,964,156), POM modified with polybutadiene (U.S. Pat. No. 4,424,307) or POM modified with a polydiene/polyalkylene oxide block copolymer (German Patent No. 2 408 487). However these mixtures do not show sufficient low-temperature impact strength for many applications.
EP 156,285 and EP 181,541 describe mixtures of POM and core shell rubber-elastic graft copolymers in which the particles have been formed from a rubber-elastic core based on a polydiene and a hard graft shell with improved low-temperature impact resistance. The aging behavior of these mixtures at elevated temperatures however is not satisfactory, which limits their application for example for automotive parts. The object of the present invention was, therefore, to provide a toughened POM composition which, compared with known systems, possesses, at temperatures down to -40.degree. C., considerably improved toughness properties, and at temperatures up to 100.degree. C. good aging properties combined with a satisfactory thermostability in the melt at temperatures up to 230.degree. C.
It has now been found that this object can be achieved, surprisingly, by employing a specially stabilized toughening component, namely a stabilized MBS core shell graft copolymer composed of a rubber-elastic polybutadiene core, one or more hard graft shells composed of polymers mainly formed from styrene, from methyl methacrylate or from mixtures thereof (amounts below 10 % of other monomers such as ethyl acrylate, or crosslinking or graftlinking monomers, may also be formed into the polymers of the shell or shells), and a special stabilization formulation. The term "hard", as known to those skilled in impact modifier technology, means that the shell polymer is a glass at room temperature or above. The special stabilization formulation consists of at least one hindered phenol, a phosphite, a sulfide, and a pH buffer system.
In contrast, the addition of common stabilizers to a core shell graft copolymer either did not improve the heat aging properties to a satisfactory extent or the thermostability in the melt deteriorated to an unacceptable extent.